New hosts and expression vectors for the production of industrially important recombinant protein are continuously being developed for the purpose of increasing production yields and simplifying down stream processes such as single-step purification using affinity tag systems. Though many expression hosts are available, Escherichia coli continues to remain one of the most frequently employed host for the mass production of various useful recombinant proteins or peptides, and many promoters such as Plac, Ptrp, Ptac, λPL, PT7 and PBAD are commonly utilized for the construction of expression vectors (Baneyx, 1999). Among these, lacUV5, tac and combined system of PT7 with lacUV5 are widely used, because the expression can easily be regulated by varying the concentration of the inducer isopropyl-beta-D-thiogalactopyranoside (IPTG, Schein and Noteborn, 1988). However, the use of IPTG precludes the use of these expression systems in pilot scale production of recombinant proteins, mainly due to the high cost and potential toxicity of IPTG (Figge et al., 1988, Kosinski et al., 1992, Bhandari and Gowrishankar, 1997, Leigh et al., 1998, Yogender et al., 2001, Wang et al., 2004). Other promoters called λPL and λPR are generally induced by a temperature shift, which can have an adverse effect on the protein folding and reduce the final yield of the product (Remaut et al., 1981).
It is known that the expression of a homologous or heterologous gene may be enhanced by replacing a promoter sequence naturally associated with that gene with a strong promoter sequence, which results in an enhanced expression of the gene at the transcriptional level (Studier and Moffatt, 1986, Gupta et al., 1999). However, ideal expression system should provide high-level expression under induced conditions and no basal expression under repressed conditions, yet should show adjustability to intermediate levels over a wide range of inducer concentrations (Rossi and Blau, 1998, Keyes and Mills, 2003). To date, only a limited number of expression system have been explored for the industrial recombinant protein production. The field of modern biotechnology is competitive and is attracting considerable interest from industrial partners outside the traditional fermentation industry, interested in the industrial applications of enzymes and other proteins. Therefore, it is not surprising that several of these partners have started to explore the possibility of using new expression systems as alternatives to those covered by patents and patent application (Staub, et al., 2002). It is in the interest of the biotechnological industry to seek new expression systems, which are easily accessible, cheap and simple to regulate. Especially, systems that are independent of the host strain, medium, and growth rate are needed. Therefore, the aim of our work was to develop a next generation of a novel expression system which fulfills most of factors to be an ideal expression system of E. coli. 
The ability to produce high biomass densities of E. coli in fermentors (Lee, 1996, Thiry and Cingolani, 2002), combined with the newly adopted regulatory genetic elements obtained from Pseudomonas putida F1 (Choi et al., 2006), renders this novel expression system extremely interesting as a potential tool for the production of recombinant proteins and of industrially important bulk chemicals. The applications of such an expression system is equally comprehensive encompassing the: (1) production of research reagents to support R&D in biotechnology and in various biological fields including proteomics; (2) production of commercial recombinant proteins (enzymes and bio-active peptides); (3) production of various biomaterials including proteinaceous and non-proteinaceous bio intermediates; (4) as a tool for metabolic engineering work.
International Patent Publication WO 2007/022623 published Mar. 1, 2007 discloses the use of regulating elements from Pseudomonas putida to enable inducible regulation of gene expression in Methylobacterium extorquens. International Patent Publication WO 2006/037215 published Apr. 13, 2006 discloses the use of cumate inducible regulating elements to enable inducible regulation of gene expression in Chinese Hamster Ovary (CHO) cells. In both of these cases, the repressor and its weak promoter are incorporated into the genome of the host cell separately from the plasmid containing the gene of interest, operator and promoter for the operator.
There is a need for a tightly regulated, inducible gene expression system in Escherichia coli. 